Sample holder

ABSTRACT

A two or three part sample holder for X-ray apparatus has a sample ring  2  and a counter ring. The sample ring  2  has two opposed surfaces and foils  8, 10  one on each surface holding a powder sample  6  within the central hole  4.  The sample ring is relatively thin, of thickness 0.2 to 4 mm. A counter ring is provided to engage with the sample ring to provide increased strength and/or stiffness.

FIELD OF INVENTION

The invention relates to a sample holder, for use in X-ray measurementapparatus, and X-ray measurement using a sample holder.

BACKGROUND ART

In order to measure powder samples in X-ray fluorescence (XRF)apparatus, it is necessary that the sample be mounted in a sample holderwhich can in turn be placed in the apparatus.

However, conventional sample holders for powder usually have asignificant contribution to the observed background intensity under thefluorescence peaks of interest caused by scattering in the sample holderwall material.

A number of different containers have been proposed for use in such

X-ray fluorescence apparatus. In particular, examples are proposed in GB2 194 636 and U.S. Pat. No. 4,037,109.

However, there remains a need for an improved method of X-rayfluorescence in which the effects of sample holders are minimised.

Further, there is a need for providing a sample holder that gives goodresults for X-ray fluorescence with a minimal volume of sample.

By way of example, during pharmaceutical research the amount ofpharmaceutical prepared can often be very low and powder samples of only0.1 g may need to be measured. The inventors tried using an existingsample cup for this powder but with such small quantities of sample theusable

SUMMARY OF INVENTION

Embodiments of the invention provide a sample holder and a method ofcarrying out X-ray fluorescence using the sample holder, the sampleholder having a counter ring and a sample ring, in which a powder sampleis held between a pair of films in a through hole in the sample ring.

The inventors have discovered that the sample holder according to theinvention can provide good XRF results even with very small quantitiesof sample. In results presented below XRF results are achieved with a0.1 g sample that are as good as when using 5 g sample in a conventionalholder.

Such results are not achievable with conventional sample holders, suchas conventional microcell holders.

The inventors have realised that even when X-rays are directed throughthe sample, and not directly irradiating the sidewalls of the sampleholder, the powder in the sample holder can scatter the X-rays towardsthe side walls of the sample holder where the X-rays can interact withthe sample holder and produce fluorescence or additional scattering. Byreducing the thickness of the sample holder significantly, scatteringand/or fluorescence from the walls of the holder, is reduced not justabsolutely, but also as a fraction of the total measured scattered orfluorescence signal. The inventors have further realised that reducingthe thickness of a sample holder reduces the stiffness of the sampleholder.

This is particularly a problem when the sample is being held in place byvery thin sheets, since the sample holder can become extremely flexible.

Accordingly, the sample holder may be a two-part or three-part sampleholder with the inner sample ring fitting into a counter ring.

In this way, a maximum area of powder sample of minimum thickness can bemeasured. The reduction in sample thickness makes the sample transparentto high-energy X-rays which significantly reduces the background due toscattering. The sample holder shapes low mass loose powders for goodmeasurements.

This in turn allows for improved lower limits of detection when makingXRF measurements.

The sample holder thickness reduction allows small amounts of materialto be handled and measured in the sample holder while remainingsufficiently stiff for keeping the sample flat and correctly located inthe X-ray fluorescence apparatus. The counter ring provides additionalstiffness than that provided by the sample ring alone. The sample holderis particularly relevant for determination of trace elements in relativehigh scattering matrix.

The counter ring may form a press fit with the sample ring.Alternatively, the counter ring may form a snap fit with the samplering. In this way, the counter ring and sample ring may conveniently bebrought together to form an integral unit without tools.

The films may be foils of thin polymer having a thickness from 1 to 50μm.

Preferably, the materials used have low atomic number, not higher than26.

The sample ring may be of polymer.

The counter ring may be of aluminium.

In the three-part arrangement, the sample holder further includes anouter ring arranged to engage the counter ring to form a unit of outerring, counter ring and sample ring, the outer ring having a through holearranged to align with the through holes of the counter and sample ringsto pass X-rays.

One of the opposed films is held between the sample ring and the counterring and the other of the opposed films is held between the counter ringand the outer ring. In this way, both films may be held between therings.

The rings may be fitted together to hold the films without the use ofadhesive.

In particular, the outer ring may form a press fit or a snap fit withthe counter ring.

BRIEF DESCRIPTION OF DRAWINGS

For a better understanding of the invention, an embodiment will now bedescribed, purely by way of example, with reference to the accompanyingdrawings, in which:

FIG. 1 shows a first half of the sample holder according to theembodiment, and

FIG. 2 shows a second half of the sample holder according to theembodiment.

FIG. 3 shows a top view of counter ring for mounting the sample holderof the first embodiment;

FIG. 4 shows a section through the counter ring of FIG. 3;

FIG. 5 shows sections through three parts of a sample holder accordingto a second embodiment; and

FIG. 6 shows a perspective view of the three parts separately; and

FIG. 7 shows the three parts mounted together with a sample powder.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS OF THE INVENTION

Referring to FIG. 1, a sample holder in the form of a sample ring, inthe embodiment a circular sample disc 2 has a central hole 4 acting as asample space for holding sample powder 6.

To retain the powder 6, opposed foils 8, 10 are provided, one on eachopposed face of the sample disc 2.

The foils are thin—for example 1 μm to 50 μm, especially 3 to 20 μm. Inthe specific embodiment shown, the foils are 4 μm to 6 μm thick. Thesample disc 2 is typically 0.5 mm to 2 mm, preferably 0.75 mm to 1.5 mmthick. The diameter of the central hole may be in the range 5 mm to 50mm, preferably 10 mm to 20 mm, and the sample space 4 typically holds 10to 200 mg of powder.

The foil is of polymer, for example nylon or Mylar. Likewise, the sampledisc 2 is of polymer, and may conveniently be of the same material. Lowatomic number materials are preferred, to avoid fluorescence.

The foil does not cover the whole of the opposed surfaces of the sampledisc 2.

In view of the materials choice and thickness of the sample disc 2, thesample disk 2 and the films are flexible which can give rise to handlingdifficulties, especially in manufacturing environments.

Accordingly, a counter ring 20 is provided that fits together with thesample ring 2 to provide additional support. The counter ring 20 is inthe form of a ring 22 with a central through hole 24 arranged tocorrespond with the central hole 4 of the sample disc 2 to allow X-raysto pass.

The counter ring has a raised outer ring 26 in which the radially innerface is arranged to engage the outer circumference of the sample disc 2to engage the sample disc.

In the embodiment shown, the counter ring 20 engages with the sampledisc 2 by a press fit. However, in alternative embodiments detent meansmay be provided to allow a snap fit.

The complete sample holder with counter ring 20 engaged with sample disc2 is sufficiently robust and in particular rigid to be able to be usedin industrial environments. The counter ring provides some measure ofprotection to foil 10 to allow the sample holder to be placed on asurface with the counter ring against the surface without damage.

In some cases, such sample holders may be used in existing equipmentreplacing existing sample holders with the same form factor butproviding improved performance.

The counter ring may be made of plastics, for example the same plasticsas the sample ring.

Alternatively, the counter ring may be made, in all or in part, ofmetal. Preferably, a metal of atomic number no higher than 26 is used.The counter ring may in particular be made of aluminium.

The sample ring need not be in the form of a circular disc, but insteadmay be for example square or rectangular. Likewise, the through holesmay be square, circular or rounded as appropriate for individual X-rayfluorescence apparatus.

Results using 0.1 g of pharmaceutical powder in a prototype sampleholder were measured and compared with those using a 5 g self standingpellet of the same material and with 5 g of powder in a conventional“P1” cup.

To compare samples, the root mean square deviation (RMS) in parts permillion were obtained using XRF for three elements, Ru, Pd and Pt, forthe three samples.

Element RMS Ru RMS Pd RMS Pt 5 g, pellet 0.29 0.18 0.33 self standing 5g, powder 0.51 0.20 0.17 in P1 cup 0.1 g powder 0.19 0.16 0.20 in holder

These results demonstrate that similar levels of deviation may beobtained using very small quantities of sample.

An alternative arrangement is shown in FIGS. 5 to 7. In this case, thereare three rings, an outer ring 50, a counter ring 52 and a sample ring54, shown in section in FIG. 5. Dotted construction lines indicatealigned parts.

The three rings are arranged to fit together with the outer ring 50having an outer retaining ring 60 on its lower side adapted to mate withan outer step 62 on the upper edge of the counter ring 52. The counterring 52 has an inner step 64 arranged to mate with an outer flange 66 onthe sample ring 54.

In use a first foil 68 is stretched across the lower surface of samplering 54 and then the sample ring 54 is mounted into counter ring 52which retains the foil stretched along the bottom surface of inner ring54. A powder sample 74 is then mounted above this foil 68 in centralcavity 70.

A second foil 72 is then mounted on the upper surface of the counterring 52 and sample ring 54 extending to the outer edge of the counterring. The counter ring 52 is then mounted to the underside of outer ring60 stretching the second foil. Thus, the powder sample 74 is heldbetween the first and second foils 68, 72. Note that in FIG. 7 the foilsare shown with thick lines for clarity but in fact the foils arerelatively thin.

In this way the powder sample can be held in place without the need forglue. In particular, in this embodiment the rings are held together by apress fit. Alternatively, a snap fit may be used.

Alternatively, for greater security, glue or other adhesive can be usedto fix the rings together.

In this arrangement all the rings can be made of the same or of acombination of different materials such as polymer and/or aluminium.

The sample holder of embodiments of the invention can minimise theexposure of holder material to the excitation X-ray beam. This reducesthe amount of holder material that can scatter and so the spectralbackground component can be reduced.

The sample holder of embodiments of the invention does not need to beabsolutely leakproof unless the sample is toxic or biologically active.

The sample holder of embodiments of the invention does not requirescrews and so is easy, simple and safe to use. The holder can avoidcontamination of the sample by humidity.

Thus, while there have been shown, described, and pointed outfundamental novel features of the invention as applied to severalembodiments, it will be understood that various omissions,substitutions, and changes in the form and details of the illustratedembodiments, and in their operation, may be made by those skilled in theart without departing from the invention. Substitutions of elements fromone embodiment to another are also fully intended and contemplated. Theinvention is defined solely with regard to the claims appended hereto,and equivalents of the recitations therein.

What is claimed is:
 1. A method of carrying out X-ray fluorescencemeasurements on a powder sample, comprising: placing the powder samplewithin a central through hole of a sample ring (2,54) having opposedsurfaces, the sample ring having a thickness of 0.2 mm to 4 mm betweenthe opposed surfaces, the central through hole having a maximum lateraldimension of 5 mm to 50 mm, providing opposed films (8,10,68,72) ofthickness 1 to 50 μm on the opposed surfaces sealing the through hole toretain sample powder in the through hole; engaging a counter ring(20,52) with the sample ring to form a unit, the counter ring having athrough hole arranged to align with the through hole in the sample ring;mounting the unit in X-ray fluorescence apparatus and passing X-raysthrough the through hole of the counter ring to the sample to carry outX-ray fluorescence measurements.
 2. A method according to claim 1,further comprising mounting an outer ring (50) to engage the counterring (52) to form a unit of outer ring, counter ring and sample ring,the outer ring (50) having a through hole arranged to align with thethrough holes of the counter and sample rings to pass X-rays.
 3. Amethod according to claim 1, wherein the counter ring engages the samplering by a press fit.
 4. A method of carrying out X-ray fluorescencemeasurements on a powder sample, comprising: stretching a first foilacross the lower surface of a sample ring having a central through hole;mounting a counter ring around the sample ring to retain the first foilstretched across the lower surface of the sample ring; filling powderinto the through hole above the first foil; mounting a second foil onthe upper surface of the counter ring and sample ring; and mounting anouter ring outside the counter ring to maintain the second foilstretched and the sample held between the first and second foils in thecentral through hole of the sample ring.
 5. A method according to claim4, wherein the foils are of thin polymer having a thickness of from 3 to20 μm.
 6. A sample holder for X-ray fluorescence apparatus, comprising:a sample ring (2,54) having opposed surfaces, the sample ring having athickness of 0.2 mm to 4 mm between the opposed surfaces, the samplering having a central through hole extending between the opposedsurfaces for receiving a sample powder, the central through hole havinga maximum lateral dimension of 5 mm to 50 mm, opposed films (8,10,68,72)of thickness 1 to 50 μm on the opposed surfaces sealing the through holeto retain sample powder in the through hole; and a counter ring (20,52)arranged to engage with the sample ring to form a unit, the counter ringhaving a through hole arranged to align with the through hole in thesample ring when engaged with the sample ring to pass X-rays.
 7. Asample holder according to claim 6, wherein the counter ring (20,52)forms a press fit with the sample ring (2,54).
 8. A sample holderaccording to claim 6, wherein the counter ring (20,52) forms a snap fitwith the sample ring (2,54).
 9. A sample holder according to claim 6,wherein the films (8,10,68,72) are foils of thin polymer having athickness from 3 to 20 μm.
 10. A sample holder according to claim 6,wherein the sample ring (2,54) is of polymer.
 11. A sample holderaccording to claim 6, wherein the counter ring (20,52) is of polymer oraluminium.
 12. A sample holder according to claim 6, further comprisingan outer ring (50) arranged to engage the counter ring (52) to form aunit of outer ring, counter ring and sample ring, the outer ring (50)having a through hole arranged to align with the through holes of thecounter and sample rings to pass X-rays.
 13. A sample holder accordingto claim 7, wherein one of the opposed films (68) is held between thesample ring (54) and the counter ring (52) and the other of the opposedfilms (72) is held between the counter ring (52) and the outer ring(50).
 14. A sample holder according to claim 8, wherein the films(68,72) are held between the rings without the use of adhesive.
 15. Asample holder according to claim 8, wherein the outer ring (50) forms apress fit or a snap fit with the counter ring (52).